1. Field of the Invention
The present invention concerns a switching mode power supply device, and more particularly a power supply device with normal operation and power saving modes.
2. Description of the Related Art
The switching mode power supply device generally employs as the reference signal an oscillation signal with a frequency as high as possible, e.g., 100 KHz, in order to increase the transformation efficiency. In the power-off or sleep mode, such an apparatus cuts off all the power sources supplying the load except the power source supplying the circuits for controlling the power saving mode, thus minimizing the power consumption.
An earlier switching mode power supply device which connected between a rectifier circuit and a load with a power saving mode, includes an oscillator, a pulse width modulation (PWM) control circuit, switching circuit and a transformer with multiple secondary coils. The transformer has at least one output voltage delivered to the load, a part of which is fed back to a feedback circuit connected to a comparator.
The oscillator generates an oscillation signal of a high frequency which serves as a reference signal for the PWM control circuit to generate a PWM signal to control the on/off operation of the switching circuit. The feed back circuit feeds a part of the output voltage of the transformer back to the comparator to detect changes in the output voltage depending on the rising and falling of the feedback voltage.
The comparator compares the output feedback signal with an inherent reference signal of the feedback circuit to provide the resultant signal for the PWM control circuit to maintain the output voltage of the transformer at a given value corresponding to the normal operation mode or power saving mode. Namely, the PWM control circuit generates a PWM signal to modify the on-duty interval in response to the oscillation signal and the feedback signal of the feedback circuit, thereby controlling the on/off operation of the switching circuit 23 to control the output voltage of the transformer.
Hence, the switching circuit carries out the switching operation according to the same oscillation signal at both the normal operation mode and the power saving mode, thus increasing the power consumption for the switching operation which reduces the advantage of the power saving mode.
The following patents each discloses features in common with the present invention but do not teach or suggest the specifically recited power supply device with a reference signal generating circuit for the power saving mode of the present invention: U.S. Pat. No. 5,737,616 to Watanabe, entitled Power Supply Circuit With Power Saving Capability, U.S. Pat. No. 5,542,035 Kikinis et al., entitled Timer-Controlled Computer System Shutdown And Startup, U.S. Pat. No. 5,745,375 to Reinhardt et al., entitled Apparatus And Method For Controlling Power Usage, U.S. Pat. No. 5,628,001 to Cepuran, entitled Power Saving Method And Apparatus For Changing The Frequency of A Clock In Response To A Start Signal, U.S. Pat. No. 5,684,681 to Huh, entitled Drive Circuit Of Switching Element For Switching Mode Power Supply Device, U.S. Pat. No. 5,812,386 to Youn, entitled Power Supply Control Method And Corresponding Circuit, U.S. Pat. No. 5,650,924 to Huh, entitled Electric Power Supply Device For A Monitor, U.S. Pat. No. 5,726,871 to Choi, entitled Power Supply Circuit With Power Saving Mode For Video Display Appliance, U.S. Pat. No. 5,657,257 to Lee, entitled Power-Supply Controller Of Computer, U.S. Pat. No. 5,808,881 to Lee, entitled Power-Supply Controller Of Computer, U.S. Pat. No. 5,646,572 to Masleid, entitled Power Management System For Integrated Circuits, and U.S. Pat. No. 5,737,616 to Watanabe, entitled Power Supply Circuit With Power Saving Capability.